The present invention relates to apparatuses for humidifying rice grains such as brown rice grains and polished rice grains and, more particularly, to an apparatus for humidifying rice grains up to a predetermined moisture content, without causing cracks to occur therein, to improve taste of boiled and cooked rice.
In general, one of important factors influencing the taste of boiled and cooked rice is a moisture content of polished rice before it is dipped in water for the purpose of being boiled and cooked. If the polished rice having the moisture content of approximately 15% is dipped in water within a cooker, no cracks occur in the surface of the polished rice. Accordingly, the polished rice having the moisture content of approximately 15% can be boiled and cooked in a whole grain configuration. The boiled and cooked rice having such whole grain configuration retains adequate glutinosity and hardness which are factors of taste, and is superior in taste. However, the polished rice having a mixture content less than 14% absorbs moisture at a considerably high rate when the rice is dipped in water. The quick absorption of moisture causes cracks to instantaneously occur in the rice grain surfaces and, soon after, the cracks are developed into the interiors of the rice grains to form splits or fractures. Water penetrates into the splits. When the polished rice is boiled and cooked under such condition, gelatinized starch flows out of the splits and adheres to the rice grain surfaces. This renders the rice grains sticky. Moreover, since the boiled and cooked rice grains are broken due to the splits, the rices grains are deteriorated in quality, and have no sufficient biting-resistance and glutinosity. Thus, in order to improve the taste, it is necessary to apply the humidifying treatment to the polished rice grains so as to have the moisture content of approximately 15%, before the polished rice grains are dipped in water for the purpose of being boiled and cooked. It is necessary to bring the moisture content of the polished rice to the level of about 15% in order also to prevent the rice from getting mold and from being decomposed. In the humidifying treatment, however, it is essential to avoid occurrence of cracks in the rice grain surfaces. The maximum limit of the rate at which the polished rice can absorb moisture under a natural condition, i.e., the maximum moisture adding rate at which the rice grains are humidified without causing cracks to occur therein is 0.3% per one hour. When moisture is added to the polished rice at a rate higher than 0.3% per one hour, the moisture exceeding the natural moisture absorbing rate of the rice grains remains on the rice grain surfaces, to thereby wet the same. If the rice grain surfaces are wetted, the surface layer of each rice grain abruptly absorbs the moisture and is expanded, to cause strain to be developed between the surface layer and the interior of the rice grain. The strain results in the occurrence of cracks in the rice grain surfaces. Even if the moisture adding rate slightly exceeds 0.3% per one hour, cracks occur in the rice grain surfaces, similarly to the case of the dipping of rice grains into water. If the moisture adding rate is considerably high, through cracks occur even in the brown rice covered with pericarp having wax property which prevents penetration of water.
In a conventional rice grain humidifying apparatus as disclosed, for example, in Japanese Patent Application Laid-Open No. 60-21315, a step of adding moisture to rice grains for a relatively short period of time of about 2 to 3 seconds and a tempering step of adding no moisture to the rice grains for a relatively long period of time of about 15 minutes are alternately repeated. In such conventional humidifying apparatus, it has generally been called that the moisture adding rate is 0.3% per one hour. Since, however, the time during which the moisture is actually added to the rice grains is 2 to 3 seconds, the net moisture adding rate during the actual addition of moisture is considerably high, i.e., on the order of 0.033% per one second. Consequently, there may be a possibility that such a risky condition occurs instantaneously that the rice grain surfaces are wetted. In view of this, it is usual to operate the humidifying apparatus at a moisture adding rate slightly lower than the desired or target moisture adding rate. This makes it difficult for the conventional humidifying apparatus to enhance the operating efficiency or performance.
In contradistinction to the above-described rice grain humififying apparatus in which the moisture is intermittently added to rice grains, a polished rice humidifying method as disclosed in U.S. Pat. No. 4,450,181 to Satake, for example, comprises the step of continuously adding moisture little by little at a rate not exceeding 0.3% per one hour. In this continuous moisture adding method, the moisture is added to the rice grains at a low rate of 0.0083% per one second and, therefore, the rice grains are humidified at the maximum limit of the natural moisture absorbing rate of rice grains without wetting the rice grain surfaces, so that no cracks occur in the rice grains. Accordingly, the continuous moisture adding method can be said to be a superior humidifying method. Since it is impossible for such humidifying method to add the moisture to the rice grains at a rate exceeding the natural moisture absorbing rate thereof, however, the time of the moisture adding treatment cannot help being prolonged or lengthened. Alternatively, if the apparatus were increased in size and capacity to enable a great amount of rice grains to be subjected at a time to the humififying treatment, the treating capapcity of the apparatus would be able to be enhanced. In such case, however, the cost of equipment would increase.